1. Field of the Invention
This invention relates to a signal processing device for processing an information signal.
2. Description of the Related Art
Known information signal processing devices include an image signal recording and reproducing system, which is arranged, for example, to frequency-modulate an image signal, to record the frequency-modulated image signal on a recording medium, to reproduce the recorded image signal from the recording medium, and to frequency-demodulate the reproduced image signal to bring it back to its original state.
FIG. 1 shows in outline the arrangement of a frequency modulator employed for the image signal recording and reproducing system mentioned above.
Referring to FIG. 1, the illustration includes an astable multivibrator 1 which is arranged to vary its oscillation frequency in proportion to an input current signal "is"; a comparator 2 which is arranged to shape the waveform of a signal outputted from the astable multivibrator 1; and an output buffer amplifier 3.
Referring further to FIG. 1, a luminance signal which has been subjected to a preemphasizing process through a preemphasis circuit (not shown) is inputted to an input terminal S1. To another input terminal V1 is inputted a DC voltage signal which is at the same potential as that of a sync tip part of the luminance signal inputted to the input terminal S1. The oscillation frequency of the astable multivibrator 1 at the sync tip part of the luminance signal is set by means of a resistor R2. An amount of frequency deviation in the oscillation frequency is arranged to be set by means of another resistor R1.
FIG. 2 shows in detail the circuit arrangement of the astable multivibrator 1 shown in FIG. 1. The operation of the circuit of FIG. 2 is described below with reference to timing charts which are shown in FIGS. 3(a) to 3(c) and 4(a) to 4(c).
In FIG. 3(a), a signal waveform shown with a full line represents a waveform obtained at the collector of a transistor Q9, while another signal waveform shown with a broken line represents a waveform obtained at the collector of a transistor Q10. In FIG. 3(b), a signal waveform shown with a full line represents a waveform obtained at the emitter of a transistor Q7, while a signal waveform shown with a broken line represents a waveform obtained at the emitter of a transistor Q8.
Let us assume that the characteristic of the transistor Q9 coincides with that of the transistor Q10. The characteristic of a transistor Q5 coincides with that of a transistor Q6. The characteristic of the transistor Q7 coincides with that of the transistor Q8. The characteristic of a transistor Q11 coincides with that of a transistor Q12. The characteristic of a transistor Q13 coincides with that of a transistor Q14. The characteristic of a resistor r1 coincides with that of a resistor r2. Let us further assume that the beta (.beta.) of each of these transistors which is IC/IB (wherein IC represents a collector current and IB a base current) is sufficiently large. Then, there are obtained the following relations: ##EQU1##
Further, when signals outputed from the emitters of transistors Q15 and Q16 are supplied to the comparator 2 of FIG. 1, with the charactersitics of the transistors Q15 and Q16 assumed to coincide with each other, the periods T1 and T2 of waveform-shaped signals outputted to the output buffer amplifier 3 of FIG. 1 become equal to each other. As a result, a frequency-modulated luminance signal is outputted with a stable duty.
However, an error .DELTA.VBE of about .+-.2 mV arises between paired transistors in a voltage VBE between the base and the emitter in general. Further, an error of about .+-.2% arises between the paired resistors. As a result, the waveform of the frequency-modulated luminance signal changes in a manner as shown in FIGS. 4(a) to 4(c).
FIG. 4(b) shows the waveform of the frequency-modulated luminance signal obtained in the event of a discrepancy between the transistors Q5 and Q6 in the characteristic of the voltage VBE between the base and the emitter. In this instance, the period TA is not equal to the period TB because iSA is not equal to iSB. However, the signal periods T1 and T2 remain equal to each other.
Meanwhile, FIG. 4(c) shows the waveform of the frequency-modulated luminance signal obtained in the event of a discrepancy in the characteristic of the base-emitter voltage between the transistors Q9 and Q10 and also a discrepancy in the characteristic of the resistance valve between the resistors r1 and r2. In that instance, TA.noteq.TB because .DELTA.VA.noteq..DELTA.VB. However, the relation of T1=T2 remains unchanged.
Meanwhile, a system like a video tape recorder or an electronic still video system is arranged to frequency-multiplex a chrominance component signal in a lower frequency band than the frequency band of the frequency-modulated luminance signal in recording and reproducing an image signal. In reproducing the signal with the system of this kind, the secondary distortion component of the frequency-modulated luminance signal causes a cross modulation component FY+FC (wherein FY represents the carrier frequency of the luminance signal and FC represents the carrier frequency of the chrominance component signal) to appear within the reproduced signal. To avoid the generation of the cross modulation component, the secondary distortion component of the frequency-modulated luminance signal must be minimized, for example, by suppressing it to such a value as -45 dB. Therefore, the signals outputted from the emitters of the transistors Q15 and Q16 of the astable multivibrator shown in FIG. 2 are arranged to be supplied to the comparator 2 shown in FIG. 1.
However, if the voltage VBE between the base and the emitter of each of the paired transistors deviates from that of the other between the transistors Q13 and Q14, between transistors Q7 and Q8 or between transistors Q15 and Q16 and if an error .DELTA.VBE is generated between the transistors paired for the inputs of the comparator 2, a DC level obtained at a level comparison point varies to cause the state of T1.noteq.T2 as shown in FIG. 3(c).
For example, if the voltages .DELTA.VA and .DELTA.VB are 500 mV and the error .DELTA.VBE between the paired transistors is .+-.2 mV, ##EQU2##
In this instance, the relation of the duty to the secondary distortion component of the frequency-modulated luminance signal is as follows: the secondary distortion is about -26 dB when the duty is 50.+-.1.6%; about -30 dB when the duty is 50.+-.1.0%; and about -35 dB when the duty is 50.+-.0.55%. Then, it is impossible to reduce the secondary distortion component of the frequency-modulated luminance signal, even if the error .DELTA.VBE is lessened by increasing the size of the transistor.
In a conceivable method for solving the problem, a capacitor is attached to the outside of the output buffer amplifier 3 and the slew rate between the capacitor and a power supply is utilized. However, the effect of this method is attainable only when the duty is below 50% if the transistor to be used is of the NPN type and only when the duty is above 50% if the transistor is of the PNP type. Therefore, this method is hardly acceptable.
In another conceivable method, a frequency band of the frequency-modulated luminance signal of the secondary harmonic system is lessened by means of a trap, a low-pass filter or the like. However, that method deprives the system of interchangeability between recording and reproduction in respect to the frequency characteristic and room for inversion.